1. Field of the Invention
The present invention relates to an apparatus for recording/reproducing information on/from an optical recording medium.
2. Description of the Related Art
In recent years, an optical recording/reproducing apparatus which can record information at a high density using a light beam or can reproduce information recorded at a high density at high speed has been developed in place of an apparatus for recording/reproducing information using a magnetic head.
According to the optical recording/reproducing apparatus, a track spirally or concentrically formed on a disk-like recording medium (to be referred to as an optical disk hereinafter) at small intervals of 1.mu. to 2.mu. is tracked by a beam spot of a light beam. In order to realize high-density/high-speed recording/reproduction, a tracking actuator having high tracking performance with respect to a high frequency and a linear motor for moving an optical head from an inner periphery to an outer periphery of an optical disk in a low-frequency region are simultaneously operated.
The papers of Optical Disk Symposium '85 report the following optical recording/reproducing apparatus.
More specifically, a displacement of a tracking actuator is directly detected by a detection sensor, and the detection sensor outputs a detection signal. The detection signal is subjected to signal processing by a phase compensating circuit and a driving circuit to be converted to a drive signal and supplied to a linear motor.
Based on the drive signal, the tracking actuator and the linear motor are simultaneously operated.
According to the conventional optical recording/reproducing apparatus, if an environmental condition is changed, an actual displacement of the tracking actuator operated under the changed condition is detected by a displacement detection sensor. Therefore, the characteristic of the tracking actuator and an inertia force acting on the tracking actuator are included in the detected displacement as information. In this case, an equivalent circuit on the basis of each tracking actuator and an inertia force correcting circuit need not be adjusted, and there is no problem of an error caused by a change in environment such as a change in temperature. However, the displacement detection sensor of the tracking actuator must be arranged so that a zero point of the displacement of the tracking actuator coincides with a zero point of the sensor output. For this reason, it is difficult to improve mounting precision of the displacement detection sensor, and a mounting position includes an error. Therefore, the displacement detection sensor includes an offset error in an initial state. When the displacement detection sensor is used, servo control is performed with reference to a target position where the displacement of the tracking actuator is not zero in practice, resulting in poor tracking performance.
Furthermore, sensors for detecting a very small displacement such as a displacement of the tracking actuator must have very high precision. In such high-precision sensors, an offset is further increased due to their high temperature dependency, thus further impairing tracking performance.